Introduction: The etiopathogenesis of childhood leukemia is not fully understood. It is suggested that endogenous viral sequences may play a role in leukemogenesis. Human endogenous retroviruses (HERVs) constitute about 8% of the human genome. Most HERVs are dysfunctional because of numerous mutations and deletions. Some HERVs, however, contain sequences capable of transcription. In patients with leukemia, the presence of antibodies against HERV-K has been identified, which could suggest increased expression of HERV-K in leukemic cells. To elucidate the role of endogenous retroviruses in leukemogenesis, studies were undertaken to assess env gene expression of HERV-K and HERV-W in childhood acute lymphoblastic leukemia (ALL) and acute myeloid leukemia (AML). Results: This study was performed in 170 children with ALL, 38 subjects with AML, and 30 healthy subjects. Expression of the env gene of HERV-K and HERV-W and the control gene ACTB was studied by real-time PCR using specific oligonucleotide primers and SYBR Green marker. Env gene expression was assessed on the basis of the absolute threshold-Ct, as well as normalized against ACTB expression and double normalized expression relative to ACTB and reference cells – normal peripheral blood lymphocytes (PBL). Env gene expression of HERV-K normalized against ACTB, as well as double normalized expression relative to ACTB and normal PBL, was significantly higher only in AML. There were no statistically significant differences in env gene expression of HERV-W normalized to ACTB in ALL and AML as compared to normal PBL. Conclusion: High normalized expression of the env gene of HERV-K in AML strongly suggests a possible contribution of this gene in the pathogenesis of AML.